[SCSI] Fix sd_probe_domain config problem

With CONFIG_BLK_DEV_SD = n and CONFIG_PM = n, you get this compile failure:

(.text+0x4f6c77): undefined reference to `scsi_sd_probe_domain'

This was introduced by

commit a7a20d10
Author: Dan Williams <dan.j.williams@intel.com>
Date:   Thu Mar 22 17:05:11 2012 -0700

    [SCSI] sd: limit the scope of the async probe domain

And happens because scsi_sd_probe_domain is conditionally defined but
unconditionally used.  Fix this by making the symbol unconditionally defined.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Dan Williams <dan.j.williams@intel.com>
Tested-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>